<computer programming> The rules one uses to indent code in a readable fashion. There are four major C indent styles, described below; all have the aim of making it easier for the reader to visually track the scope of control constructs. The significant variable is the placement of "{" and "}" with respect to the statement(s) they enclose and to the guard or controlling statement ("if", "else", "for", "while", or "do") on the block, if any.

"K&R style" - Named after Kernighan & Ritchie, because the examples in K&R are formatted this way. Also called "kernel style" because the Unixkernel is written in it, and the "One True Brace Style" (abbreviation 1TBS) by its partisans. The basic indent shown here is eight spaces (or one tab) per level; four spaces are occasionally seen, but are much less common.

if (cond)

"Allman style" - named after Eric Allman, a Berkeley hacker who wrote a lot of the BSD utilities in it (it is sometimes called "BSD style"). Resembles normal indent style in Pascal and ALGOL. Basic indent per level shown here is eight spaces, but four spaces are just as common (especially in C++ code).

if (cond)

"Whitesmiths style" - popularised by the examples that came with Whitesmiths C, an early commercial C compiler. Basic indent per level shown here is eight spaces, but four spaces are occasionally seen.

if (cond)

"GNU style" - Used throughout GNUEmacs and the Free Software Foundation code, and just about nowhere else. Indents are always four spaces per level, with "{" and "}" halfway between the outer and inner indent levels.

if (cond)

Surveys have shown the Allman and Whitesmiths styles to be the most common, with about equal shares. K&R/1TBS used to be nearly universal, but is now much less common. The opening brace tends to get lost against the right parenthesis of the guard part in an "if" or "while", which is a Bad Thing. Defenders of 1TBS argue that any putative gain in readability is less important than their style's relative economy with vertical space, which enables one to see more code on one's screen at once. Doubtless these issues will continue to be the subject of holy wars.